Fix check for F-key sequences to check for both upper- and lower-case letters, instead of twice checking for uppercase

Original nano commit:
r4581
http://svn.savannah.gnu.org/viewvc/trunk/nano/src/global.c?root=nano&view=log#rev4581

Original nano commit message:
2014-01-01 Chris Allegretta <chrisa@asty.org>
        * src/global.c (strtokeytype) - Check for lower-case 'f' for defining F-key
          sequences for consistency (previously was two checks for upper case F). Fixes
          bug 40815 reported by David Binderman <dcb314@Savannah>.