sed_i(): revamp function

This makes the usage close to actual sed and adds some security details.
The changes were applied to the places the function was used.
initial release
docs: link manual pages

Add soft-links to muw.1 for operations that don't exactly need their own
manual pages.
Makefile: add .POSIX target
Makefile: add dist and clean targets
docs: update
docs: update
docs: update
muw: add message on initialising the database
parser_definiton(): fix shellcheck warning
main(): separate preparation to its own function
notify(): add notification display support for herbe
test_connection(): use a simpler sed/grep call. Fixes #11
muw: attempt to fix SASL error on MacOS
set_msmtp(): also check the contents of msmtp config
muw: release 1.0.0-rc1
muw-delete, muw-sync: add manual pages
muw-add.1: Add section on CA Certificates
muw: change manual location on error message