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
This change keeps the trymmap() function but changes it to check whether
using mmap() can reduce the number of system calls that are required.
Nevertheless, using mmap() only reduces the number of system calls if we
need multiple read() syscalls, i.e. if the file size is > MAXBSIZE. However,
mmap() is more expensive than read() so this sets the threshold at 4 fewer
syscalls. Additionally, for larger file size mmap() can significantly increase
the number of page faults, so avoid it in that case.
It's unclear whether using mmap() is ever faster than a read with an appropriate
buffer size, but this change at least removes two unnecessary system calls
for every file that is installed.