Rename security/hashcat to security/hashcat-legacy
(Tracking upstream change)

security/hashcat has been extensively reworked and may now utilize
OpenCL to run on GPUs etc. Most people are expected to upgrade to
security/hashcat (hashcat-3.00); this port is being kept in order to
support people for whom hashcat-3.00 may not be compatible.

Add explicit dependency on math/gmp

PR: 210873
Submitted by: lifanov@mail.lifanov.com (maintainer)