What is C17 and what changes have been made to the language?

According to GCC reference, C17 is actually a bug-fix version of the C11 standard with DR resolutions integrated.

C17, a bug-fix version of the C11 standard with DR [Defect Report] resolutions
integrated, will soon go to ballot. This patch adds corresponding
options -std=c17, -std=gnu17 (new default version, replacing
-std=gnu11 as the default), -std=iso9899:2017. As a bug-fix version
of the standard, there is no need for flag_isoc17 or any options for
compatibility warnings; however, there is a new __STDC_VERSION__
value, so new cpplib languages CLK_GNUC17 and CLK_STDC17 are added to
support using that new value with the new options. (If the standard
ends up being published in 2018 and being known as C18, option aliases
can be added. Note however that -std=iso9899:199409 corresponds to a
__STDC_VERSION__ value rather than a publication date.)

(There are a couple of DR resolutions needing implementing in GCC, but
that’s independent of the new options.)

So, there are no new features included in C17.

The Cppreference (History of C) says:

Future development

C17 Next minor C language standard revision, will include all accepted C11 defect reports, but no new features.

UPDATE:

• 2018: C17 (ISO/IEC 9899:2018) (ISO Store) (Final draft) Includes the deprecation of ATOMIC_VAR_INIT and the fixes to the
  following defect reports:

[DR 400], [DR 401], [DR 402], [DR 403],
[DR 404], [DR 405], [DR 406], [DR 407],
[DR 410], [DR 412], [DR 414], [DR 415],
[DR 416], [DR 417], [DR 419], [DR 423],
[DR 426], [DR 428], [DR 429], [DR 430],
[DR 431], [DR 433], [DR 434], [DR 436],
[DR 437], [DR 438], [DR 439], [DR 441],
[DR 444], [DR 445], [DR 447], [DR 448],
[DR 450], [DR 452], [DR 453], [DR 457],
[DR 458], [DR 459], [DR 460], [DR 462],
[DR 464], [DR 465], [DR 468], [DR 470],
[DR 471], [DR 472], [DR 473], [DR 475],
[DR 477], [DR 480], [DR 481], [DR 485],
[DR 487], [DR 491]