Allow "_" in URIs

This ports 435edc3f to ojluni.

Note that this is contrary to RFC-2396, which only allows alphanumeric
characters and hyphens. We need this change because internet hosts out
there clearly don't care enough about standards.

Bug: 25897693
Bug: 18023709
Change-Id: I58559602f35e4df4b765117acb874d1411095c02