fstrm: use \retval instead of \return in a few places in the documentation
fstrm-private.h: whitespace only changes
fs_io_maybe_connect(): rewrite to use less indentation
t/test_fstrm_io_unix: add 300 second alarm timeout
Makefile.am: relink the library when the linker script is updated
fstrm_control: allow multiple "Content Type" fields for certain control frame types

STOP frames are required to have no content types fields.

START frames are required to have no more than one content type field.

other control frame types are allowed to have more than one content type
field.

a convenience function is added for matching a particular content type
against a control frame object.

this adds new functions:

    fstrm_control_add_field_content_type()
    fstrm_control_get_num_field_content_type()
    fstrm_control_match_field_content_type()

and removes the old function:

    fstrm_control_set_field_content_type()