Add bp_signup_pre_validate hook

Allows plugins to modify signups before validation takes place.

Fixes #5379

Props terraling

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7824 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Pinking shears

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7825 cdf35c40-ae34-48e0-9cc9-0c9da1808c22