profiles: allow for the default dovecot libexecdir

Though many Linux distros choose to pass /usr/lib as the libexecdir while configuring dovecot, such as Debian and Arch, others like Alpine Linux and Gentoo don't pass anything as libexecdir, allowing it to default to /usr/libexec.

Both appear to be valid. From FHS 3.0, Chapter 4.7:

Some previous versions of this document did not support /usr/libexec, despite it being standard practice in a number of environments. To accomodate this restriction, it became common practice to use /usr/lib instead. Either practice is now acceptable, but each application must choose one way or the other to organize itself.

Allow for the default libexec subdir /usr/libexec/dovecot as well as the more common /usr/lib/dovecot.

Signed-off-by: Peter Levine plevine457@gmail.com