Add comment indicating source of <sys/capability.h> on Linux

On FreeBSD <sys/capability.h> holds the Capsicum-capabilities definitions.
On Linux, this header is used for POSIX.1e capabilities, but the header isn't
installed by default.  The libcap-dev package provides the headers on
Debian-based systems, so leave a hint for anyone compiling for the first time.
Use new-style rights for mmap operations

Include mapping layer in capsicum.h for old-style systems; auto-detect
based on presence of CAP_MMAP_R macro.
Auto-detect old vs. new-style CAP_SEEK behaviour

CAP_PREAD is defined in new-style systems (to be CAP_READ|CAP_SEEK)
In kernel module test target, dump last part of kernel log
Tidy up after inotify test
Add tests for *at(2) syscalls in capability mode
Tidy up ifdefs
Sort out mkfifo/mknodat tests
Add test of kill(2) to own pid
Split out cap_getrights/cap_rights_get availability
Add initial CONTRIBUTING file
Fix typo in FreeBSD version checking
Split out cap_rights_limit() availability
Make conditional compilation more granular
Close second FD in cap_rights_limit() wrapper
Merge pull request #1 from influenza/dev

Updated to fix compilation error in linux.cc
Always expect cap_rights_limit() to fail when expanding rights
kmod: Use sys_cap_new rather than capsicum_install_fd
kmod: Don't attempt to widen rights on wrapping a capability
Merge branch 'dev' of https://github.com/google/capsicum-test into convert-to-c