util: rename parse_usec() to parse_sec() sinds the default unit is seconds

util: rename parse_usec() to parse_sec() sinds the default unit is seconds

Internally we store all time values in usec_t, however parse_usec()
actually was used mostly to parse values in seconds (unless explicit
units were specified to define a different unit). Hence, be clear about
this and name the function about what we pass into it, not what we get
out of it.