Make async encryption optional

* src/common_indep.h (opt): Add sync_enc.
* src/mail.cpp (Mail::isAsynCryptDisabled_o): Respect it.
* src/main.c (read_options): Read it.

If we have users where GpgOL crashes on send we or the
user might want to disable async encryption.

• GnuPG-Bug-Id: T3838