xdg-email: Case-insensitive run_thunderbird

I clicked the "Reply to: john doe (on-list)" link on this page and was surprised that the new message that Thunderbird opened lacked a "Cc: debian-boot@lists.debian.org" header.

The problem is that the <hfname> in the mailto URL is capitalized (Cc=), which xdg-email currently ignores.

I confirmed that a mailto: <hfname> is case-insensitive, so this patch replaces grep with grep --ignore-case and sed s/.../.../ with sed s/.../.../i in the necessary spots.