crunchgen: use realpath(3) instead of ``pwd -P''

r366466 (9c7bd4f198e1) fixed a subtle bug by stripping the trailing
'\n' appended to the output of popen("cd %s && pwd -P", p->srcdir).

Replace this cumbersome implementation with a single realpath(3) call
which avoids spawning a shell, reading from the stream with fgets(3),
and final strdup(3).

Reviewed by: arichardson, kevans
Approved by: imp
Differential Revision: https://reviews.freebsd.org/D26734