option parsing: Mark arguments that are not modified as 'const'

option parsing: Mark arguments that are not modified as 'const'

This patch does not modify any functionality but makes the option
parsing code slightly easier to read by marking arguments that are
not modified explicitly as 'const'. A new function has been added,
find_option_c(), that behaves identically to find_option(). Only
the function signature differs between these two functions.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>