Signs the given plaintext
with the keys, which were
set with gnupg_addsignkey before and
returns the signed text or the signature, depending on what was set with
gnupg_setsignmode.
Parameters
identifier
The gnupg identifier, from a call to
gnupg_init() or gnupg.
plaintext
The plain text being signed.
Return Values
On success, this function returns the signed text or the signature.
On failure, this function returns FALSE.