ngctl: Modernize code somewhat

• Replace fgets(3) with getline(3)

• Replace select(2) with poll(2)

• Avoid needlessly copying text around

• Correct use of getopt(3)

• Pick some style and whitespace nits

MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55257