The default 64K stack size used for years become too small
for modern distors (Fedora 29 and etc) where third party libraries
(such as ncurses) started to use 64K for own buffers and we get
SIGSGV early without reaching interactive console phase.

Thus we increase default size up to 256K which should fit
for common case. Later we will make this value configurable
to address arbitrary stack sizes without a need to rebuild
the code.

Note the values are switched to 4K page granularity for sake
of future modifications -- we gonna manipulate pages to
relax rss usage if OS allows.

Closes #3418
---
 src/lib/core/fiber.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: tarantool.git/src/lib/core/fiber.c
===================================================================
--- tarantool.git.orig/src/lib/core/fiber.c
+++ tarantool.git/src/lib/core/fiber.c
@@ -92,10 +92,10 @@ static size_t page_size;
 static int stack_direction;
 
 enum {
-	/* The minimum allowable fiber stack size in bytes */
-	FIBER_STACK_SIZE_MINIMAL = 16384,
-	/* Default fiber stack size in bytes */
-	FIBER_STACK_SIZE_DEFAULT = 65536
+	/* The minimum allowable fiber stack size in 4K page */
+	FIBER_STACK_SIZE_MINIMAL = 4 << 12,
+	/* Default fiber stack size in 4K page */
+	FIBER_STACK_SIZE_DEFAULT = 64 << 12,
 };
 
 /** Default fiber attributes */