Fixing list insert error under BSD sed

[sarcasticadmin]

Description

This was related to the FreeBSD port for mons and the issue opened up in the portstree bugzilla: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248632

BSD sed was unable to correctly leverage i command resulting in the following output when running mons on master 375bbba3aa700c8b3b33645a7fb70605c8b0ff0c:

 $ ./mons
sed: 1: "1iLVDS-1": command i expects \ followed by text
Monitors: 0
Mode: primary

The following change was initially suggested by @kevans91 here: Ventto/mons#30 (comment) but I did some more testing to get the following to be compliant for both GNU and BSD versions of sed.

Tests

I have added appropriate tests for both the libshlist and libshlist_unsafe variations.

$ sh tests/test_freebsd.sh 
sed is /usr/bin/sed
test: insert pass
--------------------------
sed is a shell function
test: insert pass

$ sh tests/test_unsafe_freebsd.sh 
sed is /usr/bin/sed
test: insert pass
--------------------------
sed is a shell function
test: insert pass

As for mons this also fixes the original error. I do have an issue with how command -pv xrandr is working and have to modify it to command -v xrandr but Ill open that issue in mons for discussion:

$ ./mons
Monitors:        2
Mode: primary
0:*  LVDS-1   (enabled) 

[arkhan]

hello, any update..?