Add the sqlite3_db_readonly() interface.  This is still tentative, pending
a closer look at other ideas to accomplish the same thing.

FossilOrigin-Name: 254f99ea9ff1534948bdb179e69ab0c940c87ec1

• manifest[diff]
• manifest.uuid[diff]
• src/main.c[diff]
• src/sqlite.h.in[diff]
• src/sqliteInt.h[diff]
• src/test1.c[diff]
• test/pager1.test[diff]