[PATCH 0 of 3] help: format help text using RST markup

[Olav Reinert]

This set of patches is the final move towards the goal of formatting all help
text using RST markup.

The changes affect some helper functions (which contribute parts of the help
output) to make them use RST formatting internally in a consistent manner:
Output is generated as a list of strings (each string roughly corresponding to
a line of output) marked up with RST. At the end of each method the output is
formatted and written to the ui.

Finally, the helper functions are changed to simply return the RST text they
produce, instead of outputting it themselves. The main help function is changed
correspondingly so that it collects all RST pieces before formatting and
printing all of the help text at once.

No user-visible changes are introduced by the patches, unless the patch
descriptions say otherwise.