Allow empty values

It is a legitimate use case to set empty values; to override a
default when an empty value is acceptable for instance. This patch
changes the regex in three ways: it 1) removes the requirement for
a non-whitespace terminator on a setting value, 2) makes the value
match non-greedy so that the \s*$ at the end can catch the newline
and 3) changes the \s*=\s* to [ \t]*=[ \t]* because we don't want
that to capture *any* whitespace (like a newline).