Start building Linux userspace library for Capsicum

Call it libcapsicum for now, although this is not covering the same
ground as FreeBSD's libcapsicum (which covers setting up communication
with the Casper daemon).
Update README for dependencies
Use Linux libcapsicum in tests
Add first pass at Debian packaging for libcapsicum
Use more POSIX compliant prototypes for fexecve_, execveat
Man pages for libcapsicum functions in section 3
Install man pages