grep: repeated -e and -f options

[mknos]

• Standard grep command allows -e PATTERN and -f FILE to appear multiple times[1]
• Currently if -e or -f is repeated the value is overridden and no warning or error output is produced
• Secondly, patterns from -e and -f are expected to be combined (GNU manual explains that "If [-e] option is used multiple times or is combined with the -f option, search for all patterns given.")
• Currently the documented behaviour of this grep will ignore -e pattern if -f FILE is set; the code could be improved to combine the patterns from -e and -f to follow the standard better

1. https://pubs.opengroup.org/onlinepubs/009604299/utilities/grep.html --- "Multiple -e and -f options shall be accepted"