contrib/cap_sasl.pl: import fixes from freenode's 1.5

 - fall back to Crypt::DH::GMP::Compat on Ubuntu
 - ensure that the auth file is not world-readable
 - increase timeout to 7.5s
 - update the specification URL
contrib/cap_sasl.pl: add SASL EXTERNAL
contrib/cap_sasl.pl: add SASL ECDSA-NIST256P-CHALLENGE

Largely based on the version in atheme/ecdsatool, but with some minor
fixes like checking if the tool is in $PATH...
saslserv/ecdsa-nist256p-challenge: store the authzid