WEXTRA: "Fix" missing initializers in menus.c:last_saved_pos_hints ...

... by removing all of them.  The defuault initialization is good
enough.
Pager: Fix unconditional XClearArea() calls ("if (...);").
TRAVIS-CI: Add -Wextra to gcc options ...

and remove some warnings:
  -Wno-unused-parameter
  -Wno-sign-compare
  -Wno-type-limits
  -Wno-missing-field-initializers
TRAVIS-CI: Remove trailing whitespace.