MINOR: flags/mux-h2: decode H2C and H2S flags

The new functions h2c_show_flags() and h2s_show_flags() decode the flags
state into a string, and are used by dev/flags:

  $ ./dev/flags/flags h2c 0x0600
  h2c->flags = H2_CF_DEM_IN_PROGRESS | H2_CF_DEM_SHORT_READ

  $ ./dev/flags/flags h2s 0x7003
  h2s->flags = H2_SF_HEADERS_RCVD | H2_SF_OUTGOING_DATA | H2_SF_HEADERS_SENT \
             | H2_SF_ES_SENT | H2_SF_ES_RCVD
2 files changed