Only expect openat() to return ENOTCAPABLE on systems that
police rights before cap_enter() (i.e. not on current capsicum-linux).
Abstract away differences in mount(2).

Linux and FreeBSD have radically different mount(2) syscalls,
so push the implementation into syscalls.h.
Add #define for whether SIGCHLD is generated for a pdfork()ed
child process that exits.
Allow openat() failures to be either ECAPMODE or ENOTCAPABLE.