msmtp: msmtp-scripts: Backport "Enable PROVIDES and ALTERNATIVES"

msmtp: msmtp-scripts: Backport "Enable PROVIDES and ALTERNATIVES"

Backports needed change from master for the next commit (a
combined cherry-pick and squash of required fixes to msmtp-scripts,
due to msmtp-scripts being broken on openwrt-19.07).

Use the PROVIDES mechanism so that msmtp and msmtp-nossl can be be
+depended-on and avoid generating a file level conflict.  Also use
alternatives for msmtp-mta and msmtpq-ng-mta with msmtp-mta since
we can only have one sendmail at a time.

(cherry-pick f76408af486fa7bdfd4cd8ddef2fcc76ab0fd9c4)

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>