Use C preprocessor with assembly sources (used by default when the
extension is .S). Using preprocessor includes seems a bit easier to
use with cmake.

In some configurations the build failed with the following error, when
using ".include":

/tmp/ccCLZztf.s: Assembler messages:
/tmp/ccCLZztf.s:49: Error: can't open mock_trampoline_table.s for reading: No such file or directory

The path opened by this function was:
"/proc/sys/vm/mmap_min_addr,"
Which of course always failed, due to the comma at the end
of the string.

The message about not finding capstone could be confusing,
when pkg-config is not installed, but libcapstone is.

 * Syscall logging has its own translation unit(s).
 * The syscall argument formats are stored in a table, which
makes them easier to maintain.
 * Removed some libc usages, just to make sure syscall_intercept
is not surprised by something unexpected going on in sprintf calls
(such as libc opening a file to read locales, other tracing, or whatnot).
 * Interpreting error codes returned by syscalls.

Add gen_ctags target

Improve logging of pipe/pipe2.

Fix label usage in test asm patterns

The "jmp 0b" "jmp 0f" syntax to jump backward, or forward to
numbered labels doesn't seem to be available in intel syntax
mode when using clang 5, or clang 6.

Refactor intercept template

It requires only two arguments now, instead of eleven.

The new code is somewhat cleaner, and allows for more reliable
stack unwinding.
The original values of RIP and RSP are stored on the stack
among the other registers. This removes the need for multiple
backtrace placeholder routines, with appropriate frame sizes.

Test with capstone's experimental branch

Improve capstone finding code

 * The path to the directory containing capstone's library
provided by capstone's package config file was ignored.
 * Attempting to find capstone is done before checking for toolchain
features, as that is more likely to fail than the other checks, and this
way the user is notified earlier.
 * More verbose error message when capstone is not found.

logging example -- fix a typo in a condition

Found by Coverity:
```
*** CID 1456740:  Memory - illegal accesses  (OVERRUN)
/syscall_intercept-ubuntu-16.04/examples/syscall_logger.c: 700 in print_rdec()
694     static char *
695     print_rdec(char *dst, long n)
696     {
697     	dst = print_signed_dec(dst, n);
698
699     	if (n < 0 && n >= -((long)ARRAY_SIZE(error_codes))) {
>>>     CID 1456740:  Memory - illegal accesses  (OVERRUN)
>>>     Overrunning array "error_codes" of 134 8-byte elements at element index 134 (byte offset 1072) using index "-n" (which evaluates to 134).
700     		if (error_codes[-n] != NULL) {
701     			dst = print_cstr(dst, " (");
702     			dst = print_cstr(dst, error_codes[-n]);
703     			dst = print_cstr(dst, ")");
704     		}
705     	}
```

Syscall logging improvements.

Stop using sudo on Travis.

Provide default values of GITHUB_REPO and DOCKERHUB_REPO.

Without that people who fork syscall_intercept have to set those values
in theirs Travis configuration.

Coverity integration fix + optimization

Disable reporting of coverage for tests.

Fix (f)chmod logging

Add test using vfork

There's no point in downloading/rebuilding Docker image in:
- normal (PR/merge) build for Coverity job
- Coverity build (cron/converity_scan) for non-Coverity jobs