According to git blame the trymmap() function was added in 1996 to skip
mmap() calls for NFS file systems. However, nowadays mmap() should be
perfectly safe even on NFS. Importantly, onl ufs and cd9660 file systems
were whitelisted so we don't use mmap() on ZFS. It also prevents the use
of mmap() when bootstrapping from macOS/Linux since on those systems the
trymmap() function was always returning zero due to the missing MFSNAMELEN
define. This might speed up installworld on ZFS file system and also
avoids two syscalls per installed file for UFS.
This change keeps the trymmap() function but changes it to check whether
using mmap() can reduce the number of system.