When parsing a large file with many descriptors we would repeatedly
search the remaining file for the sequence "newline + keyword + space"
and then "newline + keyword + newline" to find the start of the next
descriptor. However, if the keyword is always followed by newline, the
first search would always fail.
The optimization here is to search once whether the keyword is
followed by space or newline and avoid unnecessary searches when going
through the file.
In the long term we should use a better parser. But in the short term
this optimization will have a major impact on performance, in
particular with regard to concatenated microdescriptors.