libsysdecode: grab shmflags from sys/mman.h, add decode method

Any SHM_* flag here is (and likely will continue to be) a shmflag that may
be passed to shm_open2(), with exception to SHM_ANON. This is a prereq to
adding appropriate support to truss/kdump.

Reviewed by: kaktus (slightly earlier version)
Differential Revision: https://reviews.freebsd.org/D23733