dirfd is uninitialized at this point, and even if it were, it doesn't
make sense to use since the path is "/" -- the dirfd is ignored when
the path is absolute. Switch to AT_FDCWD to avoid all that.
```
=================================================================
==22862==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 4096 byte(s) in 1 object(s) allocated from:
#0 0x7f1fd5b12cb7 in __interceptor_malloc /usr/src/debug/sys-devel/gcc-11.2.1_p20220312/gcc-11-20220312/libsanitizer/asan/asan_malloc_linux.cpp:145
#1 0x55556abecea7 in xmalloc ../src/includes/helpers.h:64
#2 0x55556abecea7 in xasprintf ../src/includes/helpers.h:149
#3 0x55556abeb6fb in do_check ../src/rc/checkpath.c:206
#4 0x55556abeb6fb in main ../src/rc/checkpath.c:443
#5 0x7f1fd58576cf in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
SUMMARY: AddressSanitizer: 4096 byte(s) leaked in 1 allocation(s).
```
Signed-off-by: Sam James <sam@gentoo.org>
```
Direct leak of 4 byte(s) in 1 object(s) allocated from:
#0 0x7f49539534a7 in __interceptor_strdup /usr/src/debug/sys-devel/gcc-11.2.1_p20220312/gcc-11-20220312/libsanitizer/asan/asan_interceptors.cpp:454
#1 0x55d76fa66867 in xstrdup ../src/includes/helpers.h:91
#2 0x55d76fa66867 in get_dirfd ../src/rc/checkpath.c:111
#3 0x55d76fa66867 in do_check ../src/rc/checkpath.c:206
#4 0x55d76fa66867 in main ../src/rc/checkpath.c:442
#5 0x7f49536f06cf in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
```
Signed-off-by: Sam James <sam@gentoo.org>
- drop old build system
- move shared include and source files to common directory
- drop "rc-" prefix from shared include and source files
- move executable-specific code to individual directories under src
- adjust top-level .gitignore file for new build system
This closes#489.
