[gdb/contrib] Handle dot in spellcheck.sh

[gdb/contrib] Handle dot in spellcheck.sh

Add handling of '.' in gdb/contrib/spellcheck.sh.

While we're at, simplify the sed invocation by using a single s command
instead of 3 s commands.

Also introduce sed_join and grep_join.

Fix the following common misspellings:
...
bandwith -> bandwidth
emmitted -> emitted
immediatly -> immediately
suprize -> surprise
thru -> through
transfered -> transferred
...

Verified with shellcheck.