benchmark.pl: Skip ecryptfs if mount.ecryptfs is not available

Allows the benchmark to work on OS X or when ecryptfs is not
installed.
Also, introduce stopwatch_start/stop helpers to cut down copy-paste
code.
encfsctl: define a default PATH_MAX

Unless it is already defined. Fixes build errors with musl libc.
benchmark.pl: Improve help text, add Makefile target

Performing a benchmark on /var/tmp now is as easy as
"make benchmark".
Merge pull request #45 from rfjakob/next

Add PATH_MAX workaround, improve benchmark.pl