doc: dovecot.conf is 644 file permission, not 755

[flyxyz123]

Use INSTALL_DATA instead of INSTALL in doc/Makefile.am to install dovecot.conf with 644 instead of 755 file permission. After the patch, after ./autogen.sh; ./configure, doc/Makefile shows dovecot.conf is correctly installed with INSTALL_DATA which is /usr/bin/install -c -m 644 instead of INSTALL which is /usr/bin/install -c. More info about INSTALL_DATA see info make.

Not sure mailing list or github pull request is the correct place to submit the patch, so I'll try submit a patch to mailing list later on also.

Edit: I emailed the patch to the mailing list but it is being held for approval because I'm not a list member because my subscription request is pending.

Edit: More detailed writings about the BUG if you prefer:

• Issue description: /etc/dovecot.conf should be installed as 644 instead of 755 file permission.
• Actual behavior: /etc/dovecot.conf is installed as 755 file permission.
• Expected behavior: /etc/dovecot.conf should be installed as 644 file permission.
• How to reproduce: ./autogen.sh; ./configure; make; sudo make install to build and install dovecot, then you can see /usr/local/etc/dovecot/dovecot.conf is installed with 644 file permission before apply my patch.

Edit: Another way is to use $(INSTALL) -m 644 instead of $(INSTALL_DATA). This way maybe better for compatibility reason, because I'm not sure if old version of make/autoconf or non-GNU make has INSTALL_DATA or not. (I think maybe make is not very related, maybe autoconf is more related, not sure)