mount: fix -o user=foo mishandling, fix unc= generation, add prefixpath=

mount: fix -o user=foo mishandling, fix unc= generation, add prefixpath=

function                                             old     new   delta
singlemount                                         1019    1049     +30
packed_usage                                       29252   29257      +5
parse_mount_options                                  230     232      +2
mount_option_str                                     337     338      +1
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(add/remove: 0/0 grow/shrink: 4/0 up/down: 38/0)               Total: 38 bytes

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>