mkbootfs: consistency.

Move all the error reporting over to err()/errx() rather than the
current mix of errx(), fprintf()+exit(), and the homebrew die().

Also make the two "read all the lines in the file" loops match, by
copying the newer getline() style into the older fgets() loop.

Test: treehugger
Change-Id: Ia732df90588d7fd12dfe965e11c6ed72a96b098d